Rupture direction and short‐period spectral level of moderate‐sized intraslab and interplate earthquakes off the coast of Fukushima prefecture, Japan

Abstract

Rupture directions and directivity effects of moderate‐sized intraslab and interplate earthquakes off the coast of Fukushima Prefecture were estimated using seafloor S‐net records and onshore K‐NET and KiK‐net records. The azimuthal dependance of the source spectral ratio of each earthquake relative to a reference earthquake observed at the same station was modeled based on a ω−2 model assuming unilateral rupture. The azimuthal dependance of the source spectral ratio observed for most earthquakes reached a maximum at 2–3 Hz and weakened at higher and lower frequencies. The rupture directivity effect contributes to stations located up to the maximum hypocentral distance of 150 km used in this study. All four down‐dip compressional intraslab earthquakes were estimated to rupture toward the up‐dip, i.e., the land. In addition, the six interplate earthquakes were estimated to rupture either up‐dip or in the opposite direction of the strikes, i.e., between south and southeast directions. Only one interplate earthquake was estimated to rupture down‐dip, i.e., toward the land. In the case of intraslab earthquakes, the regions of large radiation patterns and large rupture directivity overlap in the onshore coastal area. Therefore, when only onshore records were used, the estimates of short‐period spectral levels were biased.